The present invention relates generally to integrated circuits, and, more particularly, to a load switch for an integrated circuit.
Load switches are used in electronic circuits to either connect or disconnect a power source to or from a load. Load switches are commonly used in battery charging circuits to charge batteries connected as the load. Typically, a load switch has an input port for receiving an input signal from the power source and an output port connected to the battery. The load switch provides the input signal as an output signal to the battery by way of the output port in order to charge the battery. The load switch includes a transistor connected between the input and output ports. When the power source is disconnected and the load switch is active, it has a very low resistance, which is in the order of tens of milliohms. This resistance is referred to as “ON resistance”. Due to the low ON resistance, a reverse current is supplied to the load switch by the battery, which discharges the battery. Further, the reverse current results in thermal power dissipation across the transistor, which increases linearly with an increase in the reverse current, such that the thermal power dissipation may exceed a thermal rating of the transistor, which could damage the load switch. Further, reconnecting the power source when the load switch is active can cause a current surge that may damage the battery.
A known technique to overcome the aforementioned problem is to compare voltages at the input and output ports of the load switch and to include a current sensing circuit in the load switch. The current sensing circuit includes transistors, operation amplifiers, and the like. When the power source is disconnected and the load switch is active, a voltage at the output port is greater than a voltage at the input port due to a voltage drop caused by the ON resistance. Since the value of the ON resistance is very low, a voltage difference between the voltages at the input and output ports is low. Hence, it is difficult to determine the disconnect status of the power source from the load. Further, due to the low voltage difference between the voltages at the input and output ports, the transistors of the current sensing circuit have poor matching with each other, which decreases the accuracy of the current sensing circuit. Hence, detection of the reverse current by the current sensing circuit is inaccurate and difficult. Further, when a level of the current of the input signal is less than a threshold value, the current sensing circuit does not accurately detect the level of the current of the input signal.
It would be advantageous to have a load switch that detects disconnection of the power source, provides a reverse current protection, and has improved current sense accuracy.